1. Field of the Invention
This invention relates to a resonance chamber of a cellular phone, and more particularly to a resonance chamber with an improved resonance effect in low frequency voices and a flattened resonance curve.
2. Description of Related Art
As a vigorous development in telecommunication industry, the demand for cellular phones has been massively increased. The cellular phone utilizes electric-acoustic transducers, such as transmitters and speakers, to transform electric signals into mechanical vibrations so as to animate acoustic messages, or to transform acoustic messages into electric signals for telecommunication purposes. The speaker is one of the most popular electric-acoustic transducers, and it is a mechanical design that can enable an input electric signal to transform into an acoustic message. Basically, according to different operation theories, the speakers nowadays can be separated into three categories: a moving-coil type, an electrostatic type, and a piezoelectric type.
In addition, as the cellular phone becomes slimmer in order to improve portability, the size of the speaker inside the cellular phone should be definitely reduced as well, and so is the inner space of the cellular phone. In the inner space of the cellular phone, a resonance chamber is particularly form to generate acoustic messages.
FIG. 1 shows a traditional cellular phone 100 having a printed circuit board (PCB) 104 housed by a shell 102 of the cellular phone 100. A displaying panel, an input device (such as a touch panel or a mini keyboard), a speaker, and a transmitter are placed in front of the PCB 104, while a battery 106 is placed behind the PCB 104. Referring to FIG. 2, which shows a cross-section view reference to A-A′ of FIG. 1, the inner space of the cellular phone 100 is divided into a first acoustic room 120 and a second acoustic room 140 by the PCB 104. The speaker 110, which is placed just behind openings 108 on the shell 102, further divides the first acoustic room 120 into a front space 122 and a rear space 124. The front space 122 and the rear space 124 are both used as resonance chambers for the speaker 110 to generate acoustic messages conveying outward through the openings 108 on the shell 102.
FIG. 3 shows a typical Helmholtz resonance chamber, which is widely applied to simulate frequency responses of a speaker system. As shown, the Helmholtz resonance chamber is a rigid-wall cavity with a narrow, short neck portion to connect to the environment. If the diameter and length of the neck portion is much smaller than the wavelength of an acoustic wave, the air inside the neck portion can be regarded as a massive block. Moreover, for the volume inside the chamber is much larger than that in the neck portion, the air inside the chamber presents a quasi spring-and-damper structure. Thus, when the frequency of an acoustic wave equals to the natural frequency of the chamber, the quasi massive-block (i.e. the air in the neck portion) inside the neck portion would be actuated to vibrate in a predetermined pattern. The actuated quasi massive-block would simultaneously rub the sidewall of the neck portion so as to damp down the dynamical motion thereof. Definitely, during the vibration described above, a respective sound is generated.
According to a calculation of Temkin in 1936, a vibration frequency of a Helmholtz resonance chamber is defined as:
                    f        =                              c                          2              ⁢              π                                ⁢                                    s                              V                ⁢                                                                  ⁢                                  ℓ                  ′                                                                                        (        1        )            
In which c stands for the speed of the sound in meters per second, s stands for the opening size of the neck portion in square meters; V stands for the volume of the resonance chamber in cubic meters; and l′ stands for an effective length in meters. In the case that the cross section of the neck portion is circular, l′=l+0.8 d, in which l is the length of the neck portion in meters and d is a diameter of the cross section in meters. It is clearly that, as the size of the resonance chamber increases, the effective resonance frequency would be lowered down.
As mentioned, the cellular phone 100 of FIG. 2 has a resonance chamber positioned in front of the PCB 104, and the size of the resonance chamber is restricted mainly by the size of the cellular phone 100. Therefore, as the cellular phone becomes slimmer, the resonance effect of low frequency voices would be worse due to a smaller resonance chamber. On the other hand, an increase in the size of the resonance chamber may result in a larger cellular phone and sacrifice the portability of the cellular phone.
Accordingly, how to enlarge the resonance chamber without changing the size of the cellular phone has become an important issue in improving voice quality of the cellular phone.